Powdered nickel-chromium based material for thermal spraying

ABSTRACT

The material according to the present invention comprises a self-flowing alloy containing particularly from 18 to 35% by weight of chromium as well as fractions of boron and silicon in a ratio of 3:4. The alloy also contains from 0.1 to 25.0% by weight of iron, except for the range from 1.5 to 6. The layers obtained by means of said material exhibit a very high resistance to temperatures, particularly also to corrosion and oxidation.

The invention relates to a powdered material comprising a self-fluxingnickel-chromium based alloy for producing, by thermal spraying andfusing, protective coatings resistant to erosion, corrosion andoxidation.

The usual spraying materials of this kind presently on the marketcomprise nickel-chromium based alloys having chromium contents between0.5 and 17.0 percent by weight. The field of use of such hard alloymaterials is the production of strongly wear resistant protectivecoatings to be exposed to corrosion and oxidation.

It appeared, however, that the oxidation resistance of those protectivecoatings is tolerably satisfactory only up to temperatures of at maximum600°-700° C. and that furthermore, under additional corrosive attack byhot gases, the life of the coatings diminishes very quickly already inthe range above 400° C. and does not meet the requirements of thepresent time.

It is an object of the invention to improve the heat-resistance ofcoatings produced from the material mentioned at the beginning and toobtain simultaneously very good resistance to erosion and/or corrosionand/or oxidation.

It has been established, surprisingly, that under specific conditions asubstantial improvement of the heat-resistance can be reached byincreasing the chromium content of the alloy beyond the usual limits.This is the more unexpected as one knows from experience that themechanical properties, in particular the hardness of the coating layer,are no more notably influenced by such an increase. The invention,however, provides the knowledge, that when properly choosing the amountsof various alloying elements with respect to each other, the diffusionof chromium towards the surface of the coating produced can be improvedand thus the heat resistance of the coating can be raised up to 900° C.

The spraying material according to the invention is characterized inthat the alloy comprised therein has the following composition, inpercent by weight

    ______________________________________                                        C             0.01-2.0                                                        Fe             0.1-25.0 except the range                                                              of 1.5-6                                              B             0.5-4.5                                                         Si            0.6-5.5                                                         Cr            18.0-35.0                                                       Ni            remainder,                                                      ______________________________________                                    

the boron content being equal to 75 percent of the respective siliconcontent, with a maximum deviation of ±10 percent.

In the present alloy, the indicated relationship between the amounts ofboron and silicon is essential. Other elements having an influence onthe rapidity of diffusion of chromium in a protective layer producedfrom such an alloy are carbon, niobium, iron and molybdenum. Regardingthe iron content of the alloy, in particular, the two partial ranges of0.1 to 1.5 and of 6 to 25 percent by weight are being taken intoconsideration. For the carbon content also a lower and a higher partialrange appear to be particularly advantageous.

Claims 2-11 accordingly indicate preferred compositions of the sprayingmaterial of the invention.

It can be noted from these compositions that more particularly for aniron content of more than 6 percent the molybdenum content should be inthe lower range, i.e. below 7 percent. On the other hand, should a highcontent of molybdenum be desired, the iron content should be low, namelyless than 1.5 percent.

In the following Table several examples of alloy compositions for thespraying material according to the invention are indicated, togetherwith, in each case, the corresponding temperature of the heat-resistanceT_(Dmax) in continuous working. The compositions are given in percent byweight.

                  TABLE                                                           ______________________________________                                                 ALLOY                                                                COMPOSITION                                                                              1      2      3     4    5    6    7                               ______________________________________                                        C          0.2    0.5    0.2   1    0.2  1.5  0.5                             Fe         0.5    1      7     1.2  1    1.5  9                               B          3.2    3      1.8   2.1  3.1  2.7  2.1                             Si         4.1    4      2.5   2.9  4    3.2  2.9                             Cr         20     24     26    30   34   19   25                              Mo         --     --     4     10   --   13   2                               Nb         --     0.2    --    0.1  0.3  --   0.1                             T.sub.Dmax 750°-                                                                         750°-                                                                         800°-                                                                        800°-                                                                       850°-                                                                       850°-                                                                       800°-                               800°                                                                          800°                                                                          850°                                                                         850°                                                                        900°                                                                        900°                                                                        850°                     ______________________________________                                    

The choice of a specific alloy composition is further made within theindicated ranges depending on the desired hardness and on the kind ofcorrosion which applies.

We claim:
 1. A powdered spraying material for producing, by thermalspraying and fusing, protective coatings resistant to erosion, corrosionand oxidation, said material comprising a self-fluxingnickel-chromium-based alloy, characterized in that the alloy has thefollowing composition, in percent by weight:

    ______________________________________                                        C             0.01-2.0                                                        Fe             0.1-25.0 except the range                                                              of 1.5-6                                              B             0.5-4.5                                                         Si            0.6-5.5                                                         Cr            18.0-35.0                                                       Ni            remainder,                                                      ______________________________________                                    

the boron content being equal to 75 percent of the respective siliconcontent, with a maximum deviation of ±10 percent.
 2. A spraying materialas claimed in claim 1, characterized in that the alloy further contains,in percent by weight, up to 2.0 of niobium and/or up to 15.0 ofmolybdenum.
 3. A spraying material as claimed in claim 1 or 2,characterized in that the silicon content is comprised between 1.8 and4.5 percent by weight.
 4. A spraying material as claimed in one of theclaims 1, 2 or 3, characterized in that the chromium content iscomprised between 20.0 and 28.0 percent by weight.
 5. A sprayingmaterial as claimed in one of the claims 1, 2, 3, or 4, characterized inthat the iron content is comprised between 6.0 and 20.0 percent byweight.
 6. A spraying material as claimed in one of the claims 1, 2, 3,or 4, characterized in that the iron content is comprised between 0.5and 1.5 percent by weight.
 7. A spraying material as claimed in claim 5,characterized in that the molybdenum content is comprised between 0.5and 7.0 percent by weight.
 8. A spraying material as claimed in claim 6,characterized in that the molybdenum content is comprised between 9.0and 14.0 percent by weight.
 9. A spraying material as claimed in any ofthe preceding claims, characterized in that the carbon content iscomprised between 0.01 and 0.3 percent by weight.
 10. A sprayingmaterial as claimed in any of claims 1 to 8 characterized in that thecarbon content is comprised between 0.5 and 1.5 percent by weight.
 11. Aspraying material as claimed in claim 10, characterized in that theniobium content is comprised between 0.1 and 1.0 percent by weight.